Membrane separation is intrinsically energy efficient. Almost all separation processes in living biological systems are performed through a membrane. At present, there exist grand challenges in both membrane materials and design for effective gas separation, particularly for CO2 separation and more particularly for CO2 separation from flue gas stream mixtures. Due to the low partial pressure of CO2 in flue gas mixtures, and the huge gas volumes, a great quantity of membrane surface area is typically needed for the separation process. While certain CO2/N2 selectivity is necessary, high permeance is an important factor for reduction of membrane surface area, module volume, and cost for making such separation devices on a commercial scale.
Polymeric membranes based on glassy polymers such as cellulose acetate, polyimide, and poly(phenylene oxide) (PPO) are commercially available products for separation of CO2/N2 and CO2/CH4 pairs. However, such polymers show decreased performance in presence of water vapor due to competitive sorption between water and permeate gases. These commercially available membranes also have low CO2 permeance under flue gas conditions.
Poly(vinyl alcohol) (PVA) membranes and other polymeric membranes have been tried. However, PVA-based membranes require separation temperatures above 100° C., which is well above flue gas temperatures. Certain thin PVAm/PPO composite membranes (molecular polyvinylamine coated with poly(phenylene oxide)) have been alleged to operate at 25 to 40° C.—within the flue gas temperature range. However, these membranes require a presence of a large fraction of moisture in the feed gas and/or sweep gas to keep the separation membrane layer wetted or swollen. As water permeance is much higher than CO2, a large volume of water must be introduced into the feed gas to keep the membrane fully wetted. For a given PVAm/PPO membrane, the CO2 permeance decreases by nearly three orders of magnitude as the feed gas relative humidity (RH) was reduced from 90% to about 25%.
A few inorganic membranes have been investigated for use as CO2 separators. It has been reported that molecules can be separated over a silicalite membrane based on differences in molecular weight as well as size or shape. The ZSM-5 (zeolite sieve of molecular porosity—5) type zeolite membranes shows only moderate CO2/N2 or CO2/He selectivity at low CO2 feed pressures, which falls short of the flue gas separation needed (CO2/N2 selectivity greater than 50).
What is needed are membranes and methods for making the same, which membranes provide improved stability, high CO2 permeation flux, and cost-effective manufacturability for commercial-scale use for separating CO2 from gas streams such as flue gases.